In Japanese Unexamined Patent Publication No. H3-181825, technology related to a photometric device for counting photons is disclosed. This device includes a photon counting circuit, a direct current component measuring circuit, and a photometric value output circuit. The photon counting circuit outputs a pulse signal having a predetermined wave height or more among pulse signals output from a photomultiplier tube. A direct current component measuring circuit detects a direct current level according to an amount of light of the pulse signal output from the photomultiplier tube and outputs a pulse signal of a pulse interval corresponding to the direct current level. The photometric value output circuit counts pulse signals output from one or both of the photon counting circuit and the direct current component measuring circuit to output a photometric value.
In Japanese Utility Model Registration No. 2567908, technology related to a light amount sensor is disclosed. This light amount sensor includes a plurality of light receiving elements, a photon counting circuit and an analog photometric circuit which process output signals from the light receiving elements, and a signal calculation circuit. The signal calculation circuit includes two correction circuits which correct outputs of the photon counting circuit and the analog photometric circuit, two weight calculation circuits, each of which multiplies an output of the two correction circuits by a weight, and an adding circuit which adds outputs of the two weight calculation circuits.